1. Field of the Invention
The invention relates to epoxy resin compositions which may be used to manufacture cured products which exhibit low dissipation factor and high heat resistance, and relates to cured products made therewith.
Priority is claimed on Japanese Patent Application No. 2002-317672, filed Oct. 31, 2002, the content of which is incorporated herein by reference.
2. Description of Related Art
Epoxy resins are widely used in the fields of electrical components and electronic components exemplified by semiconductor encapsulants, varnishes for printed circuit boards, and resist materials, since epoxy resins have excellent properties in electrical insulation, mechanical properties, and adhesive properties. In fields such as electrical components and electronic components, phenol resins such as phenol-novolak resins and amines such as dicyandiamide, and acid anhydrides curing agent are widely used as curing agents of epoxy resins.
However, high frequency bands are suitable to carry much information being tele-communicated recently in electrical components and electronic components exemplified printed circuit boards, and other electric insulation resist materials. For this reason, it is desired to present epoxy resin systems with a low dissipation factor to decrease the transmission loss in high frequency bands. However, when phenol resins, amine type curing agents, and anhydride curing agents are used, which are widely used as curing agents for epoxy resins, it was difficult to decrease the transmission loss since highly polar hydroxyl groups are formed in the curing reaction with epoxy resins.
Consequently, a curing agent which does not form hydroxyl groups during a curing reaction, for example, an aromatic polyester as the esterification reaction product of an aromatic dicarboxylic acid and an aromatic dihydroxy compound, is known from Japanese Unexamined Patent Application, First Publication No. Hei 5-51517. When this type of aromatic polyester is used as a curing agent, it is possible to minimize the formation of hydroxyl groups during the curing reaction as well as maximize the crosslinking density of the epoxy resin cured articles since this curing agent acts as a polyfunctional curing agent which has many ester groups per molecule and accordingly, the glass transition temperature is high and the material is useful as electrical insulating materials for electrical components and electronic components.
However, hydroxyl groups or carboxyl groups remain in cured articles because the aromatic polyesters at terminals of molecular chains are polar groups which are hydroxyl groups or carboxyl groups. When the hydroxyl groups remain, the hydroxyl groups markedly increase the dissipation factor. When the carboxyl groups remain, the carboxyl groups react with unreacted epoxy groups, and then hydroxyl groups are also formed. The existence of these hydroxyl groups caused a problem of increasing the dissipation factor.
As another technique for applying the ester resin which forms neither hydroxyl groups nor carboxyl groups at the terminals of curing agents for epoxy resin, a technique to lower the dissipation factor of the cured articles using an aromatic ester composition obtained by the reaction of naphthalenedicarboxylic acid and α-naphthol as curing agents for epoxy resin is known as claimed in claim 6 and disclosed in synthesis example 5 of Japanese Unexamined Patent Application, First Publication No. 2002-12650.
However, although the dissipation factor of the aromatic ester compounds is reduced, those ester compounds have only a few ester groups per molecule and thereby, the volume fraction of the inert terminal groups is increased. Since these terminal groups do not form crosslinking in the curing reaction, the concentrated terminal groups bring about decreased crosslinkng density in the cured articles that are inferior in heat resistance.